Reduction of velocity decay of fin stabilized subcaliber projectiles

Abstract: To minimise the aerodynamic drag during the flight of a fin stablised sub caliber projectile and thereby its velocity decay, the length of the section of threads (28) or group of grooves (22) arranged on the cylindrical body (10) for engaging a mating configuration on a discardable sabot (16,18,20) is limited. By locating the section of threads (28) or the group of grooves (22) aft of the point of natural transition (38,40) of the laminar boundary layer to turbulent boundary layer and adjacent the fin assembly (14), maximum benefit can be obtained from the low friction coefficient of the laminar boundary layer. A modified 60° STUB AMCE thread can be used.

Description: The present invention pertains to an armor piercing, fin stabilized discarding sabot projectile according to the preamble of patent claim 1. While proceeding along their trajectory from gun to target, such projectiles experience a reduction in velocity due to the action of aerodynamic drag. Since terminal effectiveness of such kinetic energy ammunition, typically used as armor piercing ammunition, is in the first order a function of the impact velocity at the target, a reduction of the velocity decay during flight to the target is a powerful way to improve their performance, particularly at extended target ranges. A typical fin stabilized armor piercing projectile with its discarding sabot, for example, has been described in U.S. Patent 4,901,646. Since the launched projectile is a subcaliber projectile, which implies that all its cross-sectional dimensions, including the span of the fins, are less than the bore diameter of the gun barrel, the projectile comprises a sabot; the latter is needed to support the subcaliber projectile during travel in the barrel. Upon exit from the muzzle of the barrel, the sabot is discarded and the subcaliber projectile is free to travel along its trajectory to the target. A subcaliber, fin stabilized, long rod projectile is comprised of a cylindrical columnar main body, having an essentially conical nose configuration at the front end and a stabilizing fin assembly attached to its rear. To achieve the desired performance the body of the projectile preferably consists of a high density, high strength alloy such as tungsten heavy metal or depleted uranium. Characteristically, the fin assembly consists of four or more fins systematically arranged around the main body of the subcaliber projectile. The finned subcaliber projectile is contained coaxially within the sabot, forming with the latter the projectile to be accelerated in the barrel of the gun and travelling along the axis of the gun barrel during launch. In addition to centering and radially supporting the subcaliber projectile, the sabot transmits the longitudinal acceleration caused by the gas pressure of the burning propellant to the subcaliber projectile. To allow this transmission, the subcaliber projectile and the sabot are provided with mating acceleration transfer interfaces, so that each of the generatrics of the main body comprise a plurality of grooves; to this end, the body is provided with a series of annular grooves which engage an equivalent mating section within the sabot. As an alternate interfaces with generatrics with grooves can also be achieved when substituting the annular grooves by with a length of male threads provided on the main body and mating female threads at the interior of the aluminium sabot. In prior art, the interface, i.e. the grooved or threaded portion of the subcaliber projectile extends over a major portion of the cylindrical body of the subcaliber projectile varying in length from 4 to 10 times the diameter of said body, and, in some instances, over the entire length of the cylindrical body, i.e. from the fin assembly to the shoulder of the projectile nose. A typical fin stabilized armor piercing projectile with a discarding sabot has been described in German patent Application DE-35 25 854 A1. The subcaliber projectile and the sabot are attached to each other by matching interfaces in the form of grooves. The specific design of the subcaliber projectile is unfavorable for the following reason: it is commonly known that laminar boundary layers have tendency to reduce velocity decay while turbulent boundary layers favor velocity decay. To prevent velocity decay during the flight of a projectile, in the present Case of the subcaliber projectile, it is therefore preferred that transition from laminar to turbulent boundary layers lays as far as possible from the nose of the subcaliber projectile. Laminar boundary layers are generally favored by smooth forms and surfaces, and it is obvious that in the vicinity of the grooves turbulence cannot be prevented. The grooves of the subcaliber projectile of the DE-35 25 854 extend over a lengthy portion of the subcaliber projectile and hence the velocity decay during flight will be considerable. While the annual grooves or the threaded portion is adequate to transfer launch acceleration from the discarding sabot, ist configuration thus induces adverse effects on the aerodynamic performance during flight to the target resulting in increased velocity decay and correspondingly lower impact velocities at the target. It is an object of this invention to reduce the adverse aerodynamic effects of the structural interface and resultant excessive velocity decay. SUMMARY OF THE INVENTION The aerodynamic drag of a fin stabilized subcaliber projectile consists of the drag of the projectile nose, the drag of its cylindrical main body including base drag, the drag of the fin assembly and last but not least a parasitic drag induced by either the annular grooves or the threaded section provided on the cylindrical body as required to transmit the launch acceleration from the sabot to the subcaliber projectile. The aerodynamic drag is composed of a pressure or wave drag depending on the aerodynamic configuration and a friction drag resulting from surface friction. It is known that the magnitude of the total drag is a function of the projectile velocity, the Mach number and the Reynolds number and that the latter has a pronounced effect on the nature of the boundary layer and the resulting surface friction. Most fin stabilized (long rod) subcaliber projectiles fired from 20 to 35 mm cannons have a Mach number of approximately M ≈ 4 at launch and operate over a range of Reynolds numbers Re ≈ 10 6 to 10 7 where the Reynolds number is defined as follows: Re = VxL ν where: V = Projectile velocity (m/s) L = Total Projectile Length (m) ν = kinematic viscosity of air (m 2 /s) The object of this invention concerns the reduction of the aerodynamic drag induced by the annular grooves and/or the threaded section applied to the cylindrical main body of the subcaliber projectile as necessary to transmit the launch acceleration from the aluminum component of the discarding sabot. Other components of the total projectile drag are only discussed as they influence the object of the invention. It is furthermore an object of the invention to define the length and the most effective configuration of the force-transmitting interface as well as its optimum location on the cylindrical main body. To minimize the adverse effect on the aerodynamic drag the force-transmitting interface, consisting either of annular grooves or a threaded section. should be as short as possible. Since the subcaliber projectile body preferably consists of a high strength high density metal, such as tungsten alloy or depleted uranium alloy on the one hand and the mating aluminum alloy sabot having lower strength properties on the other, the minimum number of annular grooves and/or the length of the threaded portion is determined by the latter. Preferably the structural calculation should be based on the dynamic strength properties of the aluminum alloy. Limiting the number of annular grooves or the length of threads results in a reduction of the aerodynamic pressure or wave drag. This drag component is induced by the shock waves emanating from the surface discontinuities at the exterior of the cylindrical main body. The most effective placement of the force-transmitting interface on the cylindrical body of the subcaliber projectile is a further object of the invention. As is known, the boundary layer along a conical-cylindrical body is laminar over its frontal portion provided of course, that the surface of the body is smooth and without irregularities such as steps or grooves to induce adverse pressure gradients. At a certain distance downstream from the projectile tip the laminar boundary layer becomes instable and transition to turbulent condition occurs. The critical Reynolds number at which transition occurs is in the first order a function of Mach number, M. The projectile under consideration has a value of approximately Re Trans ≈ 5 x 10 6 based on distance measured from the projectile tip at a Mach number from between 4.2 and 3.5. In the case of a subcaliber fin stabilized projectile having a length of 140mm, the transition occurs at 55 to 60 millimeters from the tip, i.e. approximately halfway between the projectile tip and the front of the fin assembly. Placement of the force-transmitting interface, i.e. its annular grooves or a section of threads aft of the station where boundary layer transition occurs, provides the advantage of making optimum use of the low boundary layer friction coefficient characteristic of laminar boundary layer. As is well known, the friction coefficient of the turbulent boundary layer encountered downstream of the transition is considerably higher. If the structural interface on the cylindrical body of the subcaliber projectile is stationed ahead, i.e. upstream, of the point of natural boundary layer transition, forced transition will occur at that point including the correspondingly higher friction coefficient as is the case in the current state of the art. Such forced transition is also accompanied by a more rapid growth of the boundary layer and may lead to separation of the boundary layer. The advantages of the invention are evident in a reduction of the velocity decay as determined by radar measurement to a range of 2000 meters and have been substantiated qualitatively by schlierenphotography of projectiles in flight. In summary, the invention reduces the decay of the projectile velocity in its flight to the target by minimizing the length of the structural interface consisting of either annular grooves or a threaded portion and its location on the cylindrical main body of the subcaliber projectile. The invention is equally applicable to spinning and non-spinning fin stabilized projectiles. BRIEF DESCRIPTION OF THE DRAWINGS The objects and advantages of the invention will become apparent from a consideration of the drawings and ensuing detailed description. Figure 1 - is a longitudinal cross-section of a fin stabilized subcaliber projectile discarding sabot assembly. Figure 2 - presents a detailed view of the annular grooves. Figure 3 - is a longitudinal cross-sectional view of a fin stabilized subcaliber projectile discarding sabot assembly provided with a threaded section. Figure 4 - presents a detailed view of the threaded section. Figure 5 - is a diagram presenting the critical Reynolds number as a function of Mach number where natural transition from laminar to turbulent boundary occurs on the cylindrical body of the subcaliber projectile. Figure 6 - is a longitudinal view of a fin stabilized subcaliber projectile illustrating the transition point between laminar and turbulent'boundary layer conditions. DETAILED DESCRIPTION OF THE EMBODIMENTS A longitudinal cross-section of the fin stabilized subcaliber projectile sabot assembly is presented in Figure 1. The subcaliber projectile comprises the cylindrical body 10, the conical nose 12, the cruciform fin assembly 14 attached at the rear. The cylindrical body 10 and the projectile nose 12 preferably consist of a high density, high strength metal such as a sintered tungsten alloy or a depleted uranium alloy. The fin assembly consists of a lower density metal such as aluminum or steel. In the case of a high velocity projectile aluminum fins require a protective coating for protection from the effects of aerodynamic heating. The fin assembly 14 commonly includes a pyrotechnic tracer 15. The fin stabilized subcaliber projectile is contained in the discarding sabot as illustrated in Figure 1. The particular sabot chosen for this illustration is designed for full spin launch as described in U.S. Patent 4,815,682 and 4,901,646. The main components of the discarding sabot are a three element aluminum base 16 contained coaxially in an injection molded plastic body 18 which serves as the bourrelet and at its rear is provided with a rotating band 20. Among others, the discarding sabot is needed to support the fin stabilized subcaliber projectile in a coaxial position within the barrel of the gun. The sabot is also required to transmit the launch acceleration to the subcaliber projectile. This is accomplished by the annular grooves 22 provided on the cylindrical body 10 and which engage a mating number of grooves within the aluminum base 16 of the sabot. In order to reduce the aerodynamic interference of the grooves during flight of the subcaliber projectile to the target, it is desirable to reduce the number of grooves as well as the space over which they are distributed to a minimum which is an embodiment of this invention. Since the strength properties of the aluminum sabot base are less than those of high density, high strength material of the subcaliber projectile the number of annular grooves is determined by the sabot base material. Therefore, it is preferable to use the dynamic strength properties of a high strength aluminum alloy such as 7075-T6 for the structural calculation. As an example for the subcaliber projectile having a length to diameter ratio of 12.5 for instance, the length over which six annular grooves are evenly spaced on the cylindrical body is not more than 1.4 body diameters. A preferable configuration for the annular grooves is shown in Figure 2. This groove profile is desirable from aerodynamic considerations since it reduces the wave and pressure drag. The form of the groove has the following characteristics: a. A load flank 24 angle of approximately thirty degrees measured from the normal to the axis. b. A release flank 25 angle of at least thirty degrees measured from the normal to the axis. c. A truncation at the crests 26 such that the depth of the groove 27 and the crest width 29 is approximately 0.3 times the groove spacing 23. A load flank 24 angle of approximately thirty degrees is beneficial in that it permits shortening the length of the acceleration transmission interface, an embodiment of the invention, by redistributing a portion of the thrust into a radial component. The radial component of the thrust is directly offset by the gas pressure impinging on the periphery of the aluminum sabot base aft of the rotating band. The release flank 25 angle should be thirty degrees or greater to reduce the magnitude of the adverse pressure gradient that occurs at this location in an attempt to prevent boundary layer separation and the concomitant increase in drag. The crest width 29 should be maximized within the constraints allowed by the strength properties of the aluminum sabot so as to provide the best conditions for the re-attachment of separated flow that occurs due to the presence of grooves. For the same reason, the groove depth 27 should be minimized, preferably no deeper than 0.065 times the projectile diameter. It should be mentioned that providing a total included angle between the load flank and release flank of sixty degrees or greater and minimizing the groove depth has a beneficial effect on sabot separation and subsequently, projectile dispersion. An alternate method to transfer the launch acceleration from the aluminum sabot to the subcaliber projectile is by means of a threaded section as illustrated is Figure 3. Such a configuration is desirable in combination with a discarding sabot containing a screw-on type aluminum sabot base. Similar design considerations for the threaded section 28 apply as were discussed for the annular groove interlace. The threaded section should be as short as possible and consist of a minimum number of threads. Preferably, the length of the threaded section should be based on the dynamic strength properties of a high strength aluminum to be used for the sabot base 30. To arrive at a low number of threads as desired to reduce the aerodynamic pressure and wave drag, the use of a high pitch is desirable. The thread pitch should be maximized within the constraints imposed by the strength properties of the material used for the mating sabot base. Based on the design consideration described above, a total thread length of approximately 1.5 diameters of the cylindrical projectile body was required. This applies for a high density metal subcaliber projectile having an approximate density of 18 g/cm 3 and a length to diameter ratio between 12-13. A STUB ACME thread or still better a modified 60° STUB thread profile as shown in Figure 4 is used having a load flank 32 and a release flank 33 of thirty degrees and a pitch 34 of approximately 0.22 projectile diameters. The 60° STUB thread profile shown in Figure 4 has been modified from the standard form by truncating the thread crest 36 minimizing the thread depth 35 and increasing the crest width 37. The aerodynamic interference from the use of the threaded configuration described above is considerably less than that experienced with a standard metric thread configuration, for instance as used in a variety of current art applications. A further embodiment of this invention is the location of the acceleration transfer interface, whether grooved or threaded, on the cylindrical main body of the subcaliber fin stabilized projectile. Projectiles of this type, also referred to as long rod projectiles, employed from automatic cannons having calibers ranging from 20 to 40 millimeters commonly are launched at muzzle velocities in the range of 1400 to 1450 meters per second. Because of their velocity and subcaliber projectile dimensions, the Reynolds numbers encountered over the trajectory from gun to target are such that the low friction coefficient of laminar boundary layer can be exploited successfully over a considerable portion of the projectile body by sound aerodynamic design. The resultant decrease in total aerodynamic drag will reduce the velocity decay from gun to target and permit higher impact velocities at the target which are important for armor penetration. Over the main body of the subcaliber projectiles, which consists essentially of a slender conical nose and a cylindrical afterbody, a laminar boundary layer will form provided that the exterior surface is smooth and free of steps and other discontinuities. Preferably a surface roughness of 0.8 micrometers or less is desirable. At a station on the cylindrical portion of the subcaliber projectile a critical Reynolds number Re Trans is attained where natural transition of the laminar boundary layer to a turbulent boundary layer will take place. The critical Reynolds number is defined as follows: Re Trans = VxL Trans ν where: V= Projectile velocity (m/s) L Trans = distance from the projectile nose to where natural transition occurs (m) ν = kinematic viscosity of air (m 2 /s) The Reynolds number indicating natural transition is a function of Mach number as shown for cone-cylinder bodies in Figure 5. From this data, the distance L Trans can be calculated (as measured from the tip of the projectile) where boundary layer transition will occur. As an example, for a fin stabilized projectile having a length of approximately 140mm and a launch velocity of 1400 m/s, the natural boundary layer transition will be initiated at approximately 60mm from the projectile tip. This is illustrated in Figure 6 showing a subcaliber fin stabilized projectile. The Figure contains the estimated points of transition initiation at launch velocity 38 and at a range of 2000 meters 40. This implies that over the forward portion of the projectile the boundary layer is laminar and beyond the point of transition will turn turbulent. It is common knowledge that the skin friction coefficient of laminar boundary layer is considerably lower than that of a turbulent boundary layer. Thus by locating the forward 42 end of the acceleration-transmitting interface to a position aft of the point of natural boundary layer transition, i.e. 38 and 40, the full benefit of maintaining the laminar boundary layer 44 and its low skin friction coefficient is assured. In contrast, the considerably larger acceleration-transmitting interfaces including their positioning in the middle portion of the subcaliber projectile as is common in current art designs, induce forced turbulence in the boundary layer and with it increased aerodynamic drag. In addition, tripping the boundary layer ahead of its natural transition location results in rapid thickening of the boundary layer and frequently boundary layer separation. In summary, embodiments of this invention, such as the shortening of the acceleration-transmitting interface to a minimum, whether consisting of annular grooves or a thread, combined with its location aft of the initiation of natural boundary layer transition are effective in reducing the aerodynamic drag and the resulting velocity decay during flight of a fin stabilized projectile along it trajectory to the target. This applies to projectiles regardless of whether they are launched at full or partial spin. As an example, a subcaliber fin stabilized projectile as shown in Figure 1, launched from a 25mm cannon at 1400 m/s, has a measured velocity decay of 244 m/s over a range of 2000 meters. A similar projectile representative of the current state of the art has a velocity decay of 300 m/s over the same range. This is a significant difference for the effectiveness of a kinetic energy armor piercing projectile. The improvement is the result of reducing the velocity decay as a function of range and not through an increase of muzzle velocity.

Claims: Armor piercing fin stabilized discarding sabot projectile, comprising a subcaliber projectile having a cylindrical main body (10) with a conical nose configuration (12) at the front end of said body (10) and a stabilizing fin assembly (14) at the rear end of said body (10); and a discarding sabot (16, 18, 20) for radially supporting and centering the subcaliber projectile in a gun barrel and for transmitting longitudinal acceleration to the subcaliber projectile during launch from the gun barrel, the subcaliber projectile having an acceleration transfer subcaliber projectile interface located on its main body, the discarding sabot having an acceleration transfer sabot interface, the two interfaces having mating sections, characterized in that - the projectile interface being located on the body aft the point of natural boundary layer transition from laminar to turbulent occurring during flight of the subcaliber projectile, said point being located at a distance L(TRANS) from the nose configuration (12) defined as L(TRANS) = Re(TRANS)ν V where L is measured in m ν is the kinematic viscosity of air, in m 2 /s Re (TRANS) is the Reynolds Number of the natural boundary layer transition from laminar to turbulent flight of the subcaliber projectile, and V is the projectile velocity, in m/s whereby maximum advantage is obtained of the low friction coefficient of laminar boundary layers. Armor piercing fin stabilized discarding sabot projectile according to claim 1, characterized in that the subcaliber projectile acceleration transfer interface is located adjacent to the fin assembly to delay forced transition of the laminar boundary layer to the turbulent boundary layer. Armor piercing fin stabilized discarding sabot projectile according to claim 1, characterized in that the subcaliber projectile is an armor piercing projectile particularly of a high density sintered tungsten alloy or depleted uranium alloy having a large length to diameter ratio. Armor piercing fin stabilized discarding sabot projectile according to claim 1, characterized in that the subcaliber projectile acceleration transfer interface is confined to a length not more than 1.5 body diameters. Armor piercing fin stabilized discarding sabot projectile according to claim 1, characterized in that each subcaliber projectile interface generatric comprises a plurality of grooves (22, 28) of a configuration to reduce pressure and wave drag, whereby the total aerodynamic drag of the projectile is reduced and the velocity decay experienced by the subcaliber projectile during flight to a target is reduced. Armor piercing fin stabilized discarding sabot projectile according to claim 1, characterized in that the subcaliber projectile interface comprises a number of annular grooves (22) and the sabot interface comprises a number of mating grooves (22) on the inside of an aluminum base (16) of the sabot, the number of the annular grooves being the minimum as determined by the magnitude of the longitudinal acceleration and the dynamic strength properties of the interlocking aluminium sabot. (Fig. 1) Armor piercing fin stabilized discarding sabot projectile according to claim 6, characterized in that each groove having a groove profile defined by a load flank (24) angle of approximately 30 degrees and a release flank (25) angle of at least 30 degrees, both angles measured from normal to the axis. (Fig. 2). Armor piercing fin-stabilized discarding sabot projectile according to claim 6, characterized in that the grooves are truncated grooves of predetermined groove spacing and with a crest width and depth of the grooves of approximately 0.3 of the groove spacing. (Fig. 2). Armor piercing fin-stabilized discarding sabot projectile according to claim 6, characterized in that the annular grooves are not deeper than 0.065 subcaliber projectile diameters. (Fig. 2). Armor piercing fin-stabilized discarding sabot projectile according to claim 5, characterized in that the subcaliber projectile interface and the sabot interface comprise mating grooves formed by profiles (28) of a thread configuration. (Fig. 3). Armor piercing fin-stabilized discarding sabot projectile according to claim 10, characterized in that the thread configuration has the highest pitch permissible under the dynamic strength properties of the aluminum base of the sabot. Armor piercing fin-stabilized discarding sabot projectile according to claim 10, characterized in that the profile of the thread of the subcaliber projectile interface is similar to that of a 60 degrees STUB thread configuration. (Fig. 4). Armor piercing fin-stabilized discarding sabot projectile according to claim 10, characterized in that the profile of the thread configuration is a STUB ACME thread configuration. (Fig. 4).

Inventors: CHRISTENSON CRAIG L, FELDMANN FRITZ K DR, GRIFFITH PAUL J, CHRISTENSON, CRAIG L., FELDMANN, FRITZ K., DR., GRIFFITH, PAUL J.
Applicants: CONTRAVES PYROTEC AG, OERLIKON-CONTRAVES PYROTEC AG
Application year: 1994
Main classification: F42B12
IPC codes: F42B14, F42B12